1. Field of the Invention
This invention relates to the control of a large class of moving element systems described as electromagnetic actuators such as solenoids, contactors, or even trains.
2. Description of the Prior Art
Electromagnetic actuators including the solenoid are also called variable reluctance actuators. In these devices we find a movable element made of either a ferromagnetic material, a magnet, or both, that has a force exerted on it by a magnetic field which has been generated by an electrical current flowing in a coil of wire. There may also be a permanent magnet in the non-moving component, and the coil then either adds to or reduces the force produced by the magnet. The coil is also typically wound on a ferromagnetic material to increase the efficiency and force.
Common examples of such items are tubular solenoids, the individual wires in many printheads, cradle relays, some types of Maglev trains, and novelty items such as globes that magnetically float without direct mechanical contact above or below their support. Loudspeakers are included in the list of systems that can be controlled with this approach. Although the motion of a loudspeaker coil is not normally controlled with a closed loop system, it is possible to do.
Although many such systems are presently controlled by feedback systems, smooth control of the motion and position of the moving element in these systems has always been a somewhat complicated combination of hardware, both mechanical and electrical. The complication arises due to the nature of the problem that these systems are trying to control. The forces controlling the closure of all of these systems are usually very non-linear. As the air gap between the magnetic materials contained in these systems decreases, the force exerted by a constant current in the coil increases. In most on/off systems of this type the result is runaway motion, with the moving element accelerating until it runs into a stop. The resultant impact can create significant noise, vibration and wear.
Many previous approaches have been tried with varying degrees of success and complexity. One example is shown by Jayawant U.S. Pat. No. 5,467,244 wherein a system is built that not only allows control of the runaway; it allows the system to control the position of the object. Other systems, such as Stupak U.S. Pat. No. 4,659,969, have also succeeded at some measure of control, with Stupak adding a Hall sensor to the system in order to monitor the magnetic flux contained in the system. An even earlier system, Gingrich U.S. Pat. No. 4,368,501 shows us how to derive the flux signal from a second winding on the device. Other systems have attempted feedback control with a variety of position sensors looking at the moving element. Seale U.S. Pat. No. 5,635,784 describes another approach to such control.
The sensing technique used is of great importance in many systems. Often the device is located in an environment that is hostile to certain types of sensors. Hall effect sensors, for example, are semiconductor integrated circuits that have a limited temperature range. The addition of extra coils to perform the sensing can have an impact on the physical size, not to mention the complexity of the additional wires. Jayawant describes a system that uses just the two wires of the existing coil, but surrounds that coil with a complicated electronic circuit to extract the signals needed for control.
What has not been realized in these previous systems is that the system they are trying to control has been overdetermined. More information is available than is needed to adequately control the motion of the moving element. Systems have attempted to drive a coil with a known signal, while at the same time measuring two parameters in the driven system. Typically the coil is driven with a voltage, and the current and the flux in the coil are measured. As noted above there are a variety of techniques used to measure both the current and the flux.
Jayawant describes measuring the inductance rather than flux in order to have two independent measurements of the system, which he felt, was necessary to compute the position of the moving element.